I have had this vacuum for 6 months now, and I use it in a large house with hardwood floors. It has to handle a lot of hair, from my wife and two cats.
I have never had a vacuum cleaner I liked before. Suction was never consistent, build quality was poor, ergonomics were bad, emptying the canister was unpleasent. I didn't vacuum often, because it was such a chore.
I LIKE vacuuming with My D4. It has tons of suction, and it keeps going even as the copius bag starts to fill. And changing that bag (which I've only just had to do) is totally trivial, takes seconds. It's also super easy to pick up and carry around, which was the worst thing about my mom's old Filter Queen.
It feels absolutely solid, and reports online are that these things are tanks.
I do have a few little niggles. One is that it can't be used standing up, which makes it not so great for quickly getting out for a spot clean. Another is that the little parking tabs for the hardwood floor foot feel like a potential point of failure (but I haven't actually experienced any issues, so maybe I'm worrying too much). Last one, I wish there was a little bit less spring tension on the power cable; a lot of the sockets in my house have very little grip, so I usually have to pull out the cord I need manually.
None of these are major issues, and do not detract from the fact that this vacuum cleaner is EFFING DOPE.